But now I want to analyze just the sub-population of males (sex==1)
and it shows that the number of obs is now 4037 (see below). How can
the number of observations increase when adding a -subpop()-
option? There are suddenly 37 extra observations. Note this
corresponds to the number of females with a missing race.

This is as it should be, since information about race is not required
on those observations outside of the subpopulation. Remember,
observations outside the subpopulation are relevant only insofar as
they reflect the variability in the proportion(s) of sampled PSUs with
at least one observation in the subpopulation.

In fact, at one point Stata did not behave properly in this regard;
this was fixed in an update to Stata 10 on 02apr2008 (see -help
whatsnew10- and search for "02apr2008").